“There could be literary thousands potentially of people like them: disenfranchised, lonely, not successful in life, angry. Those are the folks that are sometimes the hardest to figure out, and they are also the hardest to stop,” says ABC News Security Consultant Brad Garrett.

The federal buildings, with an increase in security, see nearly 1.4 million visitors each day.

Officials are not releasing a list of which locations are getting extra security.